The XP-Pen hardware reports HID "Eraser" usage (some digitizers with accompanying HID "Invert" usage, others without) for the top button. Linux' generic driver does here nothing more than forwarding the corresponding events from hardware to the userspace.
The missing Invert usage caused Linux to report Eraser as regular tip touch to the userspace, which was not intended but caused by an oversight in the kernel code. This, together with a bunch of hacks (Digimend and X11 tweaks), allowed OP to map that button to a right click.
Now, with the kernel logic fixed, these hacks do not work anymore. This is a case of xkcd://1172. The userspace solution is to remap BTN_TOOL_RUBBER to a right click for the weird stylus instead.